After ECM swap, secondary key wont start car

[2012 Legacy H6]

So I successfully reprogrammed the key to see if that was the issue but it still didn't start the car (opening door, switching from LOCK to ON 10 times and inputting the 8 digit key code twice). Sometimes when I insert that same key, the engine will begin to turn but will not turn all the way. Most of the time nothing will happen when I turn the key with the exception of lighting up the dash indicator lights. 

Subaru recently replaced my ECM and I suppose this is the cause of the second set of keys not working. How technical is it to program the keys to the new ECM?

 

This is a 2012 Subaru Legacy 3.6R

 

Thank you in advance.

Edited January 21, 2023 by 2012 Legacy H6

[Infosecdad]

As far as I know, we can program the FOB part of the key, but not the immobilizer chip that is matched with the ECM. 

I believe you would have to likely go back to the dealer to get it updated to match, hopefully they'll do it for free because they replaced the ECM previously.

[FLlegacy]

When the ecm was changed all the keys to the vehicle needed to be present for programming, A key cannot be simply added or replaced all keys must be programmed at the same time. And there is no method outside of the SSM or an equivalent to program the immobilizer chip to the vehicle.

Edited January 26, 2023 by FLlegacy